@jggimi, thank you for the clear explanation and howto. I just partioned the drive and now formatting the fat32 partition with

Code:

newfs_msdos sd0i

. It takes a long time, but I guess I just have to be patient. It doesn't matter if I do this over USB 2 or 3, since it does not actually send data over the connecting; the OS just needs to tell the drive how to format it, right?

usbdevs(8) tells me it is an EHCI device, so that's USB 2.
I have an extension USB cable between the computer and the official USB cord delivered with the disk; could that make it revert to even lower speeds?